Recent developments in cryogenic microcalorimeters have led to their increased use in many areas of analytical science and thus also the increased use of temperature sensors that can operate at cryogenic temperatures. For example, when operated below 100 mK, cryogenic microcalorimeters offer nearly 100% efficiency in the x-ray range between 100 eV and 10 keV, and further provide an energy resolution of a few electron volts. Because currently available x-ray detectors lack the capability to provide improved efficiency and resolution simultaneously, they are employed less frequently, and are often replaced by cryogenic microcalorimeters.
For instance, cryogenic microcalorimeters are now being used to improve the spectroscopy of astrophysical and laboratory plasmas, and also in enhancing the sensitivity of x-ray fluorescence methods for trace element determination. Typically it is useful to perform element determination in biological and geological specimens, as well as environmental waste samples, which often includes microanalysis using electron excitation in scanning electron microscopes or x-ray fluorescence using synchrotron radiation and proton excitation.
In a microcalorimeter, x-ray photons are absorbed and thermalized in a detector which is weakly coupled thermally to a cold bath. The resulting rise in the detector's temperature is measured with a thermal sensor, producing an electrical signal that is proportional to the x-ray energy. For operation at temperatures below 4K, these thermal sensors, or thermistors, take advantage of the strong temperature dependence of resistance in doped semiconductor crystals such as silicon or germanium.
Previously, the method of producing temperature sensors via doping crystals was accomplished by three different means: melt-doping, ion implantation, and neutron transmutation. As described below, each of these methods provide only partial solutions to the problem of developing a temperature sensor capable of performing at very low temperatures such as below about 4K.
For instance, although both silicon and germanium can be melt-doped, operating temperatures below 1K can lead to fluctuations of a few percent in the dopant concentration and fluctuations in the resistivity by more than an order of magnitude. Using ion implantation to generate temperature sensors generates high costs associated with uniformly doping large numbers of single crystals, and thus is typically only used in the most specialized and esoteric of applications. Furthermore, the reproducibility associated with ion implantation is relatively low, and the process can introduce radiation damage to the crystal.
Neutron transmutation has been used successfully to uniformly dope germanium for temperature sensor applications over a wide range of temperatures, but has drawbacks associated with methods of application of those sensors. Specifically, the methods available for cutting the thermistor sensors to the appropriate size and for attaching them to the substrate whose temperature is to be measured, are extremely limited. The use of a diamond saw to cut the germanium is expensive, and the glue used to adhere the thermistor to the substrate poses thermal conductance and volume heat capacity problems.
For the above mentioned reasons, a highly sensitive temperature sensor with a performance that is not limited by size is needed, one that can be manufactured reproducibly, uniformly, and in large quantities at a relatively low cost. The present invention provides a technology for making such a temperature sensor.